Effects of seaweed replacement by hydrolyzed soybean meal on growth, metabolism, oxidation resistance and body composition of sea cucumber Apostichopus japonicus

• Hydrolyzed soybean meal can be used in diet of sea cucumber as seaweed replacer
• Appropriate inclusion level of hydrolyzed soybean meal improves growth and metabolism
• Adding 8.03%–8.15% hydrolyzed soybean meal achieve the optimal nitrogen supply

A feeding trial was conducted to assess effects of seaweed replacement by hydrolyzed soybean meal (HSBM) on growth performance, diet utilization, metabolism and body composition of sea cucumber Apostichopus japonicus. Six iso-energetic (11.39 ± 0.08 kJ/g) and isoprotein (13.62 ± 0.05 g/kg) diets were formulated by replacing 0%, 20%, 40%, 60%, 80% and 100% seaweed protein with graded HSBM levels of 0%, 4.09%, 8.18%, 12.27%, 16.36% and 20.45%. The seventh diet (S12.27%) used 12.27% soybean meal (SBM) instead of 12.27% HSBM. Triplicate tanks (40 sea cucumber per tank) were fed one of seven diets for 45 days. The results showed that growth performance of sea cucumber increased with the increase of HSBM levels from 4.09% to 12.27% while decreased as the HSBM level increased to 20.45% (P < 0.05). Sea cucumber fed diets with HSBM level from 8.18 to 16.36% have higher daily feeding intake (P < 0.05), while those fed diet with HSBM level of 20.45% have lower feed efficiency compared with the control treatment (P < 0.05). Apparent digestibility coefficients of dry material and crude protein of sea cucumber fed diets with HSBM levels from 8.18% to 20.45% were significantly higher than that of control group (P < 0.05). High inclusion of HSBM (16.36% and 20.45%) significantly reduced intestine-body wall ratio, and moderate to high HSBM levels (8.18–20.45%) reduced viscera-body wall ratio and intestine-viscera ratio (P < 0.05). Alkaline protease activity was elevated by dietary addition of HSBM at level of 8.18% but suppressed at level of 20.45% (P < 0.05). Amylase activity was also elevated significantly by addition of HSBM levels from 12.27% to 20.45% (P < 0.05). Intestinal glutamic-pyruvic transaminase (GPT) and glutamic-oxaloacetic transaminase (GOT) activities significantly increased in sea cucumber fed diets with HSBM levels from 4.09% to 12.27% but GPT activity of intestine and body wall decreased in those fed diets with HSBM levels from 16.36% to 20.45% compared with the control group (P < 0.05). Intestinal superoxide dismutase (SOD) activity increased in sea cucumber fed diets with HSBM levels from 8.18% to 12.27% (P < 0.05), but the activities of SOD and glutathione peroxidase of body wall showed significant declines at high HSBM levels (16.36% and 20.45%) (P < 0.05). The contents of Ash, Mg, Ca, Zn and Se of body wall significantly increased in sea cucumber fed diet with HSBM level of 8.18% (P < 0.05). Fe content of body wall significantly increased in sea cucumber fed diets with HSBM level from 12.27% to 20.45%. Sea cucumber fed diet with 12.27% HSBM have inferior growth performance, diet utilization, oxidation resistance and metabolism compared those fed diet with 12.27% SBM (P < 0.05). Present study indicated that sea cucumber could utilize effectively HSBM as a seaweed powder replacer. Based on growth performance, we can conclude that the optimum dietary levels of HSBM for sea cucumbers is 8.03–8.15% (replacing 39.25–39.86% seaweed).Statement of relevanceOur paper provided an idea of optimizing the feed formula of sea cucumber by adding hydrolyzed soybean meal in sea cucumber culture. This optimized formula could improve diet utilization and promote growth of sea cucumber. More importantly, replacement of seaweed powder with hydrolyzed soybean meal could reduce the use of seaweed powder due to unstable supply and increased price.